Recompiled 3.8.3 kernel without SMT scheduler (smt (hyperthreading) scheduler). Only enabled "multi-core scheduler support" and system seems a little faster (automatic group scheduling enabled). Tried same setting without automatic group scheduling, but normal desktop use seemed somewhat slower; didn't try single application performance however. Wasn't able to test with both scheduler's off (xorg startup issue after re-installing kernel=what the heck? Happened when I reverted back settings too).

I wonder if it does anything on Intel CPUs that are not Pentium4.
I've got a i5-3210M CPU here, so it's not a Pentium 4 - it's a dual core but manages 4 threads at once. So is this the kind of hyperthreading SMT support enables?_________________Kali Ma
Now it's autumn of the aeons
Dance with your sword
Now it's time for the harvest

Intel and SPARC are SMT/hyperthreading, i.e. 1 real core has a few extra registers added to make thread switching cheaper. The kernel SMT option is designed with those chips in mind. The AMD chips are built the other way around; it makes sense to treat them as non-SMT cores, because technically it's the same thing as having two real 386 cores that share a 387 co-processor.